Dunes
Center Manual
What
is a dune?
A dune is any accumulation of sand-size, windblown
materials.
“...dunes can be considered as deformable obstructions to air
flow; they are free to move, divide, grow and shrink to changing
conditions” - Arthur L. Bloom, a noted geomorphologist
Where
are dunes found?
Dunes occur where there is a large supply of sand, wind to move
it and a place where it can accumulate. Dunes are found on coasts,
near rivers and in desert basins.
Why
are dunes special?
Dunes are unique environments and support rich habitats. They are
studied for practical purposes such as travel, dune stabilization,
flora, fauna and archaeology. Dunes inspire artists and spiritualists.
Dunes are fun places.
“... you seem to belong in endless time and there’s something
about the dunes, always the same slant, the same curve. Places you
go down deep and come up....peace and quiet. You have a curving
beauty that always is changing, with long slanting lines to match
the curves in the sand ... Here I feel, no wonder I lived here,
it was terrific.” -
Ellwood Decker, an artist who lived in the dunes (a Dunite) recorded
in 1991 by Huell Howser for the “California Gold” video
series #202 (available at local libraries).
Processes of sand movement:
Sand Ripples: small moving sand bedforms:
Notes:
the difference between ripples and dunes is that ripples are moving
sand bedforms responding to wind; dunes are large enough features
that they act on and alter wind to create conditions for deposition
of sand.
Dune Landforms in Uni-directional Winds:
Internal Dune Bedding:
Wind flow over isolated dune:
Aeolian erosion features downwind of dune:
Evolution
of a yardang
S
A N D
Definition
of SAND:
mineral particles smaller than a granule and larger than a silt
grain, or a loose aggregate of such particles
Sand
size range:
0.0625-2.0 mm (0.0025-0.08 in)
or 1/16th - 2 mm ( 1/400th - 1/12th inch )
Composition
of sand at Pismo Dunes (Tresselt 1960):
85-90 % quartz (silica)
10-15 % feldspars (albite, pink orthoclase)
0.25-1.5 % heavy minerals (hematite, leucoxene, magnetite, chlorite,
biotite, glaucophane, garnet; rare - rutile and calcite)
Sand
at Guadalupe Dunes (Parsons 1999):
mineral matter: >99 % medium sand, <1 % silt, and no clay
Dune
soil at Guadalupe Dunes (Parsons 1999):
slightly acid to neutral yellowish brown sand to sandy loam; surface
soil (root zone, or soil A horizon) is 40-54 cm thick in Transverse
and Lobate Dunes, up to 80 cm thick in Parabolic Dunes; Parabolic
Dunes have soil with increased pH, slight pedologic development,
and incipient lamella.
SANTA
MARIA BASIN
Geology:
This triangular sedimentary basin extends over 20 miles along the coast and
up to 90 miles inland, and covers over 1,000 square miles; initial subsidence
occurred about 18 million years ago; the synclinal trough creates a subsiding coast
that acts as a catchment for sediment; marine sediments contain
3,000-8,000 meters of sediment; transitioned from marine to terrestrial
landscape about 3 million years ago; present landscape since about
450,000 years ago.
Offshore:
The Continental Shelf extends up to 25 miles west of the present shoreline; formed by wave
erosion during times of lower sea-level (sea-level during large
glacial epochs over 100 m lower than today’s sea-level); the
100 m contour, approximating sea-level about 10,000 years ago, is
about 9 miles west of the present shoreline; the 10 m contour, approximating
sea-level about 5,000 years ago, is about a half mile out.
Annual
Sand Budget (Bowen and Inman 1966):
offshore input to beach: 76,000 cubic meters
stream input to beach:
- San Luis Obispo Creek- 6,000 m3
- Arroyo Grande Creek - 10,000 cubic meters
- Santa Maria River - 46,000 cubic meters
- sand input to Pismo-Guadalupe Dunes: 95,000 cubic meters
Twentieth
Century Scientists in the Dunes
|
Time
Period
|
World
Wide
|
California
|
| 1940’s
– 50’s |
Ralph
Alger Bagnold, North Africa |
|
| 1960’s
|
Edwin
D. McKee, White Sands, global sand seas
|
Peter
Tresselt, sand at Pismo Dunes
|
|
William
S. Cooper, Pacific Coast dune fields
|
|
A.J.
Bowen and D.L. Inman, sand budget of central coast
|
|
1970’s
–80’s
|
Edwin
D. McKee, aeolian bodies of the world
|
W.R.
Dupre, Monterey Bay dunes
|
|
Richard
A. Davis, coastal dunes
|
Donald
Lee Johnson, Vandenberg, regional dune chronology
|
|
Authur
L. Bloom, dune geomorphology
|
|
Ronald
Greeley, planetary aeolian bodies
|
|
1990’s
|
|
Anthony
R. Orme, Pacific Coast, Santa Maria area, Morro Dunes and
Mussel Rock Dunes
|
|
Robert
P. Sharp, Nipomo Dunes
|
|
Lawrence
E. Hunt, Santa Maria area
|
Selected
References for Geology and Geomorphology
of the Pismo-Guadalupe Dunes
|
Tresselt,
Peter
1960
|
1960
Recent Beach and Coastal Dune Sands at Pismo Beach, California,
Masters thesis, Department of Geology, University of California,
Los Angeles.
Particle size analysis of beach and dune sands
at 7 traverses in the Pismo Dunes; roundness; heavy mineral
analysis; limited conclusions. |
|
Cooper,
William S.
1967
|
Coastal
Dunes of California, Geological Society of America Memoir
104, Boulder, CO.
First comprehensive widespread geomorphology
study of West Coast dune fields; detailed maps of all dune fields;
depicts Flandrian 1 and 2, and pre-Flandrian dunes; photographs. |
Department
of Water Resources
1970
|
Sea-Water
Intrusion: Pismo-Guadalupe Area, California Department of
Water Resources Bulletin No. 63-3, Sacramento.
Subsurface well-data on aeolian, recent (Holocene)
alluvium, and Plio-Plei. Paso Robles FM (ancient terrestrial
alluvium) aquifers; discussion of water chemistry. |
Smith,
Kent A.
1976 |
1976
The Natural Resources of the Nipomo Dunes and Wetlands,
California Department of Fish and Game and US Fish and Wildlife
Service Coastal Wetlands Series #15, Sacramento.
Early study; good maps and description of general
area. |
Orme,
Anthony R., and Vatche P. Tchakerian
1986 |
Quaternary
Dunes of the Pacific Coast of the Californias, pp. 149-175
in W.G. Nickling, ed., Aeolian Geomorphology, Allen and
Unwin, London.
Good descriptions of Pismo-Guadalupe Dune Complex;
also San Quintin Basin, Baja; sand grain study; uses surficial
morphology, vegetation, grain-size distribution and morphology,
and weathering mineralogy to separate dune sands of varying
age. |
Orme,
Anthony R.
1992
|
Late
Quaternary Deposits Near Point Sal, South-Central California:
A Time Frame for Coastal-Dune Emplacement, pp. 309-315 in
C.H. Fletcher III and John F. Wehmiller, eds., Quaternary
Coasts of the United States: Marine and Lacustrine Systems,
Society of Economic Paleontologists and Mineralogists (Society
for Sedimentary Geology) Project #274 Quaternary Coastal Evolution,
Tulsa, OK.
Best dune stratigraphic study; south end of Pismo-Guadalupe
Dunes. |
Sharp,
Robert P., and Allen F. Glazner
1993 |
Chapter
6: Nipomo Dunes: An Ice Age Sand Lobe, pp. 57-64 in Geology
Underfoot in Southern California, Mountain Press Publishing
Company, Missoula, MT.
Suggest old sands on Nipomo Mesa are about 160,000
years old. |
| Geology
: |
|
Woodring,
W.P., and M.N. Bramlette
1950 |
Geology
and Paleontology of the Santa Maria District, California,
US Geological Survey Professional Paper 222, Government Printing
Office, Washington, D.C. |
Hall,
Clarence A., Jr., and Charles E. Corbato
1967 |
Stratigraphy
and Structure of Mesozoic and Cenozoic Rocks, Nipomo Quadrangle,
Southern Coast Ranges, California, Geological Society of
America Bulletin vol. 78, pp. 559-582. |
|
Alterman,
Ina B., Richard B. McMullen, Lloyd S. Cluff, and D. Burton
Slemmons
1994
|
Seismotectonics
of the Central California Coast Ranges, Geological Society
of America Special Paper 292, Boulder, CO.
|
|
Page,
Benjamin M., George A. Thompson, and Robert G. Coleman
1998
|
OVERVIEW:
Late Cenozoic Tectonics of the Central and Southern Coast
Ranges of California, Geological Society of America Bulletin
vol. 110, no. 7, July 1998, pp. 846-876, Boulder, CO.
|
FREQUENTLY
ASKED QUESTIONS:
What is the origin of the sand in the Pismo-Guadalupe Dunes?
Sand originates from the weathering of rocks in mountain watersheds
and seacliffs. Sand is transported to the ocean by streams and currents.
Waves wash sand onto the beach. Wind blows sand inland, where it accumulates
as dunes.
How long have the Pismo-Guadalupe Dunes existed?
The dunes we see were formed during the past 3,000 years and continue
to form today. Older dunes also exist. Some, known as the Old Dunes
(at Los Osos, Grover Beach, Oceano, Nipomo Mesa, Casmalia Hills
and VAFB), have eroded dune shapes and are 3,000-9,000 years old.
Still more ancient dunes have been eroded down to sand plains and
are up to 200,000 years old (Nipomo Mesa, Casmalia Hills, VAFB).
The oldest sands are a part of the Orcutt Formation that are up
to 500,000-1,000,000 years old (best exposed near Orcutt).
Is the sand entirely silica or are there other components?
About 85-90% of sand at Oceano is quartz (or silica). Feldspars
(albite, pink orthoclase) make up most of the remaining 10-15%.
Heavy minerals comprise about 0.25-1.50 % of sand.
Are significant quantities of sand being added from hill erosion
to the east or is the sand we have pretty much it?
Measurements of the littoral sand budget (sand moving along beaches)
in the Pismo-Guadalupe Dunes area indicate the coast receives annually
about 62,000 cubic meters per year of sand from nearby streams (damming of streams
may reduce this amount), but the majority of sand (76,000 cubic meters per year)
originates from offshore sand deposits on the continental shelf.
About 95,000 cubic meters per year of sand accumulates in the dune field each year.
Why doesn't the wind blow sand further inland and form dunes
further east?
Dunes move. Historical rates are 0.6 meters per year (2 feet per year). Dunes will
continue to move inland unless the effects of wind are diffused,
the supply of sand decreases, vegetation overgrows dunes or an
obstruction blocks inland movement. The parabolic dunes are thought
to have moved up to 5 miles before becoming stabilized.
How deep is the sand in the Pismo-Guadalupe Dunes and what lies
beneath it?
Although the tallest dunes are about 160 feet in elevation, most of
the dunes are about 100 feet high. The dunes have accumulated on
former valley surfaces (at or just above sea-level) and are underlain
by mostly stream alluvium. Thus, the dunes are approximately 100
feet thick.
How
does the shape of San Luis Bay affect the dunes?
San Luis Bay is a “log-spiral” coast formed downcoast
from resistant headlands (Point San Luis and the Pecho Coast [Diablo
Canyon area]). As waves refract around the headlands, their orientation
to the beach changes. Beaches off the dunes have long parallel wave
fronts (waves have long crests and break all at once, as opposed
to a good surfing wave with a progressive break). These parallel
waves push sand onto the beach, whereas in areas where waves strike
the coast obliquely, waves transport sand downcoast (littoral transport).
What will happen to the Pismo-Guadalupe Dunes over the next five
hundred (or five thousand) years assuming continued current practices?
We don’t know. Over the next 50 years, many people predict
consequences of global warming will result in a faster rise in sea
level that could result in inundation of shorelines. This would push
the dunes further inland in valley areas open to wind, but would
likely destroy dunes in the Oceano area, as the escarpment of Nipomo
Mesa would halt dune migration and winds would be blocked by the
hills at Grover Beach.
Will continued revegetation in the dunes change the composition
of the dunes (because of decaying organic material) and eventually
result in soil that will support a different ecosystem?
Sand is sediment. When colonized by plants and animals, soil is
created. Natural soils typically form in 50-500 years. Soils in
the Transverse and Lobate Dunes are only poorly developed; they
are 200-1,800 years old. Soils in the Parabolic Dunes are about
2,000 years old and have greater pedologic development. Rejuvenated
dunes (from coastal changes or land-use changes) can re-start the
clock. Artificial restoration efforts may accelerate the formation
of soils in sand, but are presumably only successful where vegetation
can compete with blowing sand, so these efforts probably only mimic
natural processes.
Why is Oso Flaco Lake located where it is?
Oso Flaco Lake is formed over an abandoned channel of the Santa
Maria River. This channel is thought to have been active before
the 1860’s, but was abandoned when the channel meandered southward
on the valley plain to its current location. Much of the ancient
channel filled in with sediment; however, the channel still acts
as a conduit for groundwater in the valley alluvium and helps support
the lake.
What are the sources of the water in Oso Flaco Lake?
Water is supplied to Oso Flaco Lake by two groundwater systems:
Groundwater in Santa Maria Valley and groundwater from aquifers
in the dunes.
Why doesn't the water sink into the ground rather than remain
as a lake?
Fine grain valley alluvium, which underlies the lake and dunes,
forms an aquiclude, or layer of earth that does not allow water to pass through.
Water in the lake is perched on top of the valley sediments.
Does Oso Flaco Lake ever have some sea water in it?
Fresh water sources support the lake and are sufficient to keep
out sea water.
How old is Oso Flaco Lake?
The modern lake formed after the Santa Maria River changed course
in the 1860’s. An earlier lagoon may have existed for many
thousands of years.
Sand
Dune Glossary
Aeolian: pertaining to wind; especially said of rocks, soils
and deposits (such as loess, dune sand) whose constituents were
transported (blown) and laid down by atmospheric currents, or of
landforms produced or eroded by wind, or of sedimentary structures
(topsets, dune crossbeds) made by the wind, or of geologic processes
(such as erosion and deposition) accomplished by the wind. Named
for the Aeolian Islands (also Lipari Islands) near Sicily where
Ulysses encountered Aeolus, keeper of the winds (Greek).
Albedo: that fraction of total incident light reflected in
all directions.
Anchored dune: sand dune covered with vegetation whose movement
is arrested.
Angle of repose (rest): maximum angle at which loose material, such
as talus or sand, is stable. For sand, angle of repose is 30-34¾.
Arms: in parabolic dunes, flanking ridges extending upwind;
formed when dune moved over surface leaving behind vegetated arms
as the dune advanced; arms generally lack slipfaces.
Avalanche: see sand avalanche.
Avalanche bedding: steeply inclined bedding in barchan and
related dune forms produced by avalanche of sand down the slipface
of the dune.
Barchan (or barcan, barchane, barkan, barkhan): crescent-shaped
sand dune formed, generally, by moderate winds and with a moderate
sand supply. Windward slope is gentle and lee slope or slipface
is at the angle of repose of dry sand.
Barchanoid dune: a transitional dune form between transverse
dunes with long crests, and individual lobate or barchan dunes.
Generally, long transverse crest degraded into a scalloped crest.
Blowout: trough or saucer-shaped hollow formed by wind erosion
in pre-existing dune or sand deposits.
Blowout dune: sand accumulation derived from blowout troughs
or depressions, especially where the accumulation is of large size
and rises to considerable heights above the source area.
Brink (of dune): sharp break in slope to the slipface.
Chevron dune: V-shaped dune formed by strong winds of constant
direction where wind is in conflict with vegetation.
Clay: particles of mineral matter 1/256 mm and smaller.
Climbing dune: sand piled up against a cliff or mountain
slopes by the wind.
Complex dunes: sand accumulation formed by the intersection
of two or more dune types.
Compound dune: sand accumulation formed by the intersection
of two or more of the same dune type.
Coppice dunes: small sand hummocks or mounds that have accumulated
around plants.
Creep: slow, forward motion of grains that are too large
to be lifted by the wind, whose motion is produced by impact from
saltating grains.
Crescentic dune: a barchan dune.
Crest (of dune): highest part of dune.
Crossbeds: internal landform sedimentary features.
Deflation: removal of loose, granular particles by the wind,
caused by less material coming upwind than leaving downwind.
Deflation armor: surface mosaic of pebbles or rocks, a desert
pavement.
Deflation basin: hollow formed by removal of sand and dust
by aeolian action and commonly with a rim of resistant materials
surrounding the depression.
Deposition: piling up of loose, granular particles by the
wind, caused by more material coming upwind than leaving downwind.
Dune: any accumulation of sand-size, windblown materials.
Dune slacks: low lying area between dune belts; typically
associated with patches of moist soil where low elevation surface
intersects groundwater; also called interdune areas.
Eolian: see Aeolian
Eolianite: all consolidated sedimentary rocks derived from
wind-deposited sediments.
Erg (or ergh): vast region covered with sand.
Falling dune: sand accumulation sloping at the angle of repose
of dry sand at the base of a cliff, on a mountain slope, or in a
valley, which is formed as sand is blown off a mesa top or over
a cliff face or steep slope.
Fish hook dune: dune consisting of a long sinuous sigmoidal
ridge forming the shaft and a well defined crescent forming the
hook.
Fixed dune: non-migratory dune fixed by vegetation.
Flow separation: when the primary flow near the surface no
longer continues to follow the surface contour because of sudden
change in contour or because of insufficient flow momentum to allow
flow to proceed into a region of increasing pressure.
Foreset beds: deposits formed on the slipface of a dune.
Fulgurite: long tube of fused sand resulting from lightning
strikes into sand dunes.
Gibber: residual fragment or lag gravel (Australia).
Gibber plain: a rock floored plain.
Goz: a long sand ridge (Egypt).
Helical flow: downward, circular wind flow in lee of an individual
dune crest.
Helical score: re-entrant (wind eroded depression, cavity
or pit) created by helical flow.
Horns: in barchan dune, flanking downwind portions of dune
along edges of slipface. In parabolic dunes, horns, better referred
to as arms lacking slipfaces, point upwind.
Horseshoe vortex: the U-shaped vortex wrapped around a boundary
layer obstruction. The arms of the vortex trail downwind and can
persist for many obstruction diameters. The vortex is created from
the upwind laterally oriented vorticity being forced to “pile
up” on the windward side of the obstruction and being stretched
and re-oriented in the downward direction in the trailing arms of
the vortex.
Interdune areas: low lying area between dune belts; typically
associated with patches of moist soil where low elevation surface
intersects groundwater; also called dune slacks.
Keel: sand ridge between two adjacent flutes.
Lag deposit: residual cover of pebbles and gravels remaining
after finer material has been removed by the wind.
Lamella: soil features forming thin (<1 cm; usually <5
mm) near horizontal, wavy, kinky dark bands in Oceano soil subsoils
(soil C horizon; usually over 40 cm below surface; can be many meters
below surface). Lamella are thought to form in zones of illuviation
in soil related to the soil wetting front, and are often enriched
in organic matter, clay and dark minerals such as manganese.
Lee: sheltered, or the part of an object turned away from
the wind.
Lee dune: general term for a dune formed to the lee of an
obstruction.
Loam: a mixture of sand, silt and clay in varied amounts.
Lobate dune: a single, lobe-shaped dune breaking a long dune
crest; generally forms downwind of a blowout.
Longitudinal dune: any linear dune ridge, usually more or
less symmetrical in cross-profile, which extends parallel to the
direction of the dominant wind.
Migration: movement of a dune due to transfer of sand from
the windward to leeward side.
Mineral matter: natural sedimentary clasts and grains in
soil and sediment (i.e., sand, silt and clay).
Oceano soil: distinctive soil formed in Old Dunes (2,000-9,000
years old) and marked by the formation of subsoil lamella.
Organic matter: remains of plants, animals and microfauna
or decayed organic remains in soil; one of two building blocks of
soil (organic matter and mineral mater), organic matter is the most
important constituent giving soil its characteristics.
Parabolic dune: long scoop shaped hollow of sand with points
tapering to the windward and whose groundplan approximates the form
of a parabola.
Precipitation ridge: name for large slipfaces noted in Oregon
Dunes where sand falls from suspension in large showers during active
periods. Eroded, stabilized large sand dunes in the Monterey Bay
Dunes have been interpreted as ancient precipitation ridges (paleo-climate
indicators).
Re-entrant: negative topographic features created by aeolian
conditions such as concavities, pits and/or grooves in dunes and
sandy areas.
Ripple: small scale aeolian or aqueous bedform of regularly
repeated pattern.
Ripple index: ratio of wavelength to wave height. Range in index
is great; many ripples have index of around 18.
Rising dune: sand accumulation blown up against the face
of a steep slope, mountain flank or cliff.
Saltation: bouncing or leaping movement of rock particles, usually
sand sized, carried by the wind or water currents.
Sand: particles of mineral matter 1/16-2 millimeters in diameter.
Sand avalanche: movement of a large mass of sand down a dune
face when the angle of repose is exceeded, or when the dune is disturbed.
Sand dune: ridge or pile of sand resulting from aeolian action.
Sand sheet: sand area marked by extremely flat surface broken
only by small sand ripples.
Sediment: solid fragmental material, mostly mineral matter,
transported and deposited by wind, water or ice, chemically precipitated
in a solution, or secreted by organisms, that forms in layers in
loose unconsolidated form, e.g., sand, mud, till, etc. Examples:
sand in dunes, alluvium in stream valleys or alluvial fans, submarine
deposits.
Silt: particles of mineral matter 1/256-1/16 mm in diameter.
Silt particles tend to be planar or platy in shape.
Singing sand: sand and sand dune which when disturbed if
walked upon or when sand slides down the slipface, sets up a musical
tone or humming or booming sound. Also known as acoustical, barking,
booming, musical, roaring, sonorous, sounding, whispering and whistling
sands.
Slipface: steep face on the lee side of a dune which stands
at the angle of repose of sand.
Soil: a living system formed by the interaction of life and
natural materials at and near the earth’s surface. The building
blocks of soil are organic matter (remains of plants, animals, microfauna)
and mineral matter (sand, silt, clay, loam).
Stabilized dune: a dune which has become overgrown with vegetation,
anchoring sand and stopping dune migration.
Star dune: stationary dune, with radial buttresses extending
in many directions, which develops in an area where the wind blows
from multiple directions.
Stoss: windward side of an object such as a dune.
Suspension: mixture of fine particles within a liquid or
gas for which the terminal speed of the particles is small compared
to characteristics of turbulent fluctuation speed.
Threshold wind speed: value of wind speed at a given reference
height, at initiation of grain motion.
Topsets: crossbeds created by dune accretion; also created
in part by moving sand ripples on a surface.
Transverse dune: asymmetrical dune ridge transverse to the
dominant direction of sand transporting winds, in which the leeward
slope is at the angle of repose of sand and the windward slope is
comparatively gentle.
U-shaped dunes: see parabolic dunes.
Ventifact: wind-modified object (such as a rock exposed to
wind and abraded by wind blown particles).
Vortex: any flow with closed streamlines, usually such that
fluid motion takes place in circular or near-circular paths about
some common axis.
Vortex pit: re-entrant (wind eroded depression, cavity or
pit) created by vortex flow.
Vorticity: mathematically, a vector equal to the curl of
the flow velocity vector. A fluid parcel may translate, deform
and rotate. Physically, the vorticity at a point in the fluid is
equal to twice the rate of rotation at the point.
Windward slope: slope of a dune facing the upwind direction;
generally gently sloping (10-15¾); leads up to dune crest.
Yardang: streamlined, aerodynamically shaped elongate hill
oriented parallel to the wind; term is derived from the Turkistani
word yar meaning ridge or steep bank from which material is being
removed.
